In order to properly measure the displacement, the object must be completely submerged, however in the diagram the wood is floating. So the measured displacement will only be a fraction of what it actually is.
OILRIG:
Oxidation is loss (of electrons)
Reduction is gain (of electrons)
so...
The first one is an oxidation half-equation as the Sn loses electrons;
The second one is a reduction half-equation as the Cl₂ gains electrons